1. Field of the Invention
This invention relates to planar microwave antennas and, in particular, to a rear cavity structure for such antennas.
2. Description of Related Art
Planar arrays of resonant slot elements combined with a suspended stripline feed network have been proposed as a potentially low-cost alternative to other microwave antennas. These arrays have the advantage that they are flat and slim as opposed to traditional dish reflectors. One class of planar arrays which is suitable for circular polarization uses an array of "slots" (actually in the form of circular or square apertures) with a resonant back structure to enhance the forward radiation and to provide good bandwidth and return loss from the individual feeds to each of the slots. In this form of antenna the back structure consists of an array of individual cavities, each aligned with one of the slots. Considerations such as operating frequency, antenna efficiency and sidelobe performance determine the size and spacing of the slots and their associated cavities.
For low-cost mass production the cavity structure should be pressed from a sheet of suitable metal such as steel or aluminium. However, owing to the constraints imposed by performance requirements, it has been found that such a structure cannot be manufactured without recourse to specialist pressing techniques or the use of expensive alloys.
Microwave theory has indicated that it is possible to replace the individual cavities such that some or all of the slots are served by a common cavity. Taken to its extreme this means that the array of cavities can be replaced by a single flat reflecting plate. A dual-slot antenna having this design is described in European Patent Publication No. 0 252 779 - see FIG. 1 therein. Here a single flat back plate 14 constitutes a common cavity for the two arrays of slots. The use of a flat back plate has the disadvantage that the antenna structure is less rigid and "sagging" of the slot array sheets means that the tight tolerances required for good microwave performance cannot be assured during manufacture or use. The use of insulating spacers to maintain the separation of the slots from the cavity plate can reduce the tolerance problems, but makes mass assembly of the antenna slow and undesirably complicated. Furthermore, in operation the flat back plate can support a number of degenerate waveguide modes which cause drop-outs or resonances in the microwave performance. In an alternative design described in the same European patent specification--see FIGS. 10 and 11 therein--it is proposed to replace the simple back plate with a structure in which an array of individual cavities--one for each pair of slots--is pressed out of a metal sheet. Although this form of back structure overcomes the problem of supporting the slot array to prevent sag, it requires expensive precision engineering in the formation of the individual cavities. Further, since the individual cavities are required to be larger in diameter than the slots, it imposes a restriction on the minimum separation of the slots and hence on the antenna performance.